bencode: Fix marshalling of []byte(nil)

Also fixes #293.

bencode/encode.go

@@ -159,15 +159,15 @@ func (e *Encoder) reflectValue(v reflect.Value) {
 		}
 		e.writeString("e")
 	case reflect.Slice:
-		if v.IsNil() {
-			e.writeString("le")
-			break
-		}
 		if v.Type().Elem().Kind() == reflect.Uint8 {
 			s := v.Bytes()
 			e.reflectByteSlice(s)
 			break
 		}
+		if v.IsNil() {
+			e.writeString("le")
+			break
+		}
 		fallthrough
 	case reflect.Array:
 		e.writeString("l")

bencode/encode_test.go

@@ -53,6 +53,7 @@ var random_encode_tests = []random_encode_test{
 	{[4]byte{1, 2, 3, 4}, "li1ei2ei3ei4ee"},
 	{nil, ""},
 	{[]byte{}, "0:"},
+	{[]byte(nil), "0:"},
 	{"", "0:"},
 	{[]int{}, "le"},
 	{map[string]int{}, "de"},

peer_protocol/pex_test.go

@@ -15,3 +15,11 @@ func TestUnmarshalPex(t *testing.T) {
 	require.EqualValues(t, 1286, pem.Added[0].Port)
 	require.EqualValues(t, 0x100*0xb+0xc, pem.Added[1].Port)
 }
+
+func TestEmptyPexMsg(t *testing.T) {
+	pm := PexMsg{}
+	b, err := bencode.Marshal(pm)
+	t.Logf("%q", b)
+	require.NoError(t, err)
+	require.NoError(t, bencode.Unmarshal(b, &pm))
+}